Early CT scanners that were used for scanning the brain had only a single detector and a single pencil beam X-ray source. The source and detector were repeatedly translated across the head rectilinearly a short distance and then rotated to acquire the plurality of views required to obtain an image. The early scanners required about 300 seconds to complete a 180 degree scan. Historically the next advance in scanners, known as "second generation" scanners also used a two motion system, but improved the data acquisition speed to below 20 seconds through the use of an array of detectors and a fan beam X-ray source. Twenty seconds is a normal breath holding period; and thus, the second generation tomographic scanners managed to reduce motion blurring and artifacts due to respiration.
Third generation CT scanners also known as rotate-rotate scanners used fan beam X-ray sources and an array of detectors that rotated simultaneously about the subject. The scan time of the third generation scanners in general is under 5 seconds. The fourth generation CT scanners also use a fan beam X-ray source that rotate within a circle of stationary detectors occupying a full 360 degree circle around the subject. Hence, the successive generations of CT scanners increased the scan speed to decrease the scanning time. Each generation used more detectors in the detector arrays and thereby substantially increased the costs of the system. The increased number of detectors, of course, increased the spatial resolution. Thus in successive gnerations the speed of operation and the cost of the scanners were increased while the spatial resolution was improved.
One method used to increase the speed of the earlier scanners; i.e., first generation scan CT scanners was the use of tandem detectors to obtain dual slices in a single scan. This practice was discontinued when detector arrays were used. Thus, after the scan speed improvement of the second generation it was generally assumed by those skilled in the art that there was no longer a need to acquire data for two slices simultaneously.
An important factor mitigating against the simultaneous acquisition of dual slice data in a single scan is that to accomplish such dual slice imaging it is necessary to increase the number of detectors. Each detector, of course, normally requires a separate channel with all of the front end electronics and hardware to support the detector. Hence, each added detector substantially increases the cost of the tomographic equipment. Thus, while dual slice equipment saves time it does substantially increase the cost and in the past has increased artifacts caused by the scanning operation. Accordingly, those skilled in the art have not used simultaneous dual slice features since about the time of the introduction of the fan beam; i.e., the second generation scanners and certainly it is not known that any have been used in third generation scanners even though there have been suggestions for using simultaneous dual slice acquisition with fourth generation machines. See, for example, an article entitled "Theoretical Possibilities for a CT Scanner Development" by Dr. D. P. Boyd, which was published in Diagnostic Imaging in December, 1982.
In general, the speed of scanning of computerized tomographic systems has increased from something like 5 minutes to less than a second. The increased speed has led to improved image quality; because among other things, of a reduction of motion caused artifacts. In addition the spatial resolution has improved due to increased computer power, and the number and density of the detectors. In the article, the problem of the additional cost of the detectors and hardware required for dual slice acquisition is addressed by the suggestion of the use of a plurality of X-ray sources displaced from each other in the Z direction rather than detectors displaced from each other in the Z direction. The Z direction is transverse to the longitudinal direction of the detector array or where the detector array is arcuate, the longitudinal direction of the top view planar projection of the detector array.
As witnessed by the fourth generation scanners, however, those skilled in the art are still searching for methods and apparatus to further decrease motion caused artifacts in addition to increasing the throughput and decreasing the exposure of the subject to radiation.
Accordingly, an object of the present invention is to provide a dual slice data acquisition system for use in third generation rotate-rotate computerized tomographic scanners.